ya i got puffers mixed up, but the size of the guppy depends on what tipe you get most common guppy males usually range from 1-1.5 while females range from 2-2.5
The one gallon per a inch of fish is usually the common rule to follow

i think in this case since they just wants to just setup a tank with something in it, so for them its a perfect rule and go to his picture that is in his first post in the pic shows THREE guppies which from the look of it is a good size for them

just because you think it's a perfect rule does NOT mean that it's the most healthy for the fish. I just think 3 gallons is a bad idea for a group of guppies. they sell goldfish tanks at 2.5 gallons and are they a good idea? nope.

ok iv heard some of the older members say its ok to divide a 10g into four to three sections to house bettas, but its not ok for a couple of guppies to live in, they wouldnt make small fish tanks if fish wernt suppose to go in them

Eatmice2010, you're providing wrong info.
You're a newbie to this hobby, aren't you? Beat that to 3 experienced fish keepers who have experience over 5 years. (?) Guppies and puffers are different. Guppies are active fish, they don't like staying in one spot. Puffers are more of an explorer. They won't mind going in circles or so. Guppies will be stressed in small tanks due to their high activity level. Don't even forget about the ammonia being produced. Marketers make small tanks because they know people won't buy big tanks. They want small, cute little tanks for cute little fish. You're basically saying that goldfish deserve to be in small tanks.

They make small fish tanks probably because it is cheaper to make them, but they can sell them at a premium price. In stores here, I have seen very small tanks that cost the same amount as a much larger one.

Bettas are quite inactive fish compared to those that live in groups or schools. My guppies were constantly on the move whizzing around their tank, chasing each other and spawning. My splendens on the other hand would usually slowly drift around over the course of the day, maybe stopping to have a rest every now and then.

For me, there is more than stocking a tank than just how many fish will fit in size wise. You also have to take into consideration the activity level requirements of the fish in question. I mean you wouldn't chuck three danios into a 3 gallon tank and they are only the size of guppies.